High-frequency quenching may be employed as quench hardening on a ring-shaped member of steel such as a bearing ring of a rolling bearing. This high-frequency quenching has such advantages that equipment can be simplified and heat treatment in a short time is enabled, as compared with general quench hardening of heating the ring-shaped member in a furnace and thereafter dipping the same in a cooling liquid such as oil.
In order to simultaneously heat an annular region, to be quench-hardened along the circumferential direction of the ring-shaped member in the high-frequency quenching, however, an induction heating member such as a coil for induction-heating the ring-shaped member must be arranged to face this region (for example, see Japanese Patent Laying-Open No. 59-118812 (Patent Document 1)). In a case of quench-hardening a large-sized ring-shaped member, therefore, there are such problems that a large-sized coil therefore and a large-capacity power source for this coil are required and the production cost for a quenching apparatus increases.
As a method to avoid such a problem, the following method has been proposed. That is, a coil is arranged to thee part of a circumferential surface of a ring-shaped member and is relatively rotated along the circumferential direction of the ring-shaped member so as to form an annular heated region heated to a temperature of not less than an A1 point, and then the whole of the heated region is simultaneously cooled to a temperature of not more than Ms (for example, see Japanese Patent Laying-Open No. 2011-26633 (Patent Document 2)).